Optical fiber transmission techniques are being used more and more for long-distance telecommunications since such techniques provide communications that are reliable at high data transmission rates.
However, whatever the type of modulation used (e.g. solitons or RZ modulation), pulses conveyed by fibers are subjected to deformation during propagation.
To solve this problem, long-distance optical fiber telecommunications system include inline repeater devices that resynchronize and reshape the transmitted pulses. Such repeater devices are uniformly distributed on such optical fiber lines, e.g. once every 50 km.
Various active control dispositions have also been proposed for optical transmission lines in order to increase range, data rate, or spacing between repeaters of optical fiber communications systems.
An effective method consists in inserting a modulator on the line with the control signal of the modulator varying periodically at the low frequency of the data rate (e.g. 20 GHz for 20 Gb/s). Associated with spectrum filtering, inline modulation puts pulses, and in particular soliton type pulses, back into shape, reducing jitter and noise.
Such inline modulation is applied in particular to long-range terrestrial or submarine systems.
For example, in early 1986, document JP-A-62189830 made proposals for an optical repeater device of the type shown in accompanying FIG. 1 and comprising an input optical amplifier 10, an isolator 11, and a bistable semiconductor laser 12 mounted in cascade on the line, together with a resistor 13 for detecting changes in the current injected into the input amplifier 10 under the influence of the light applied to the input of said amplifier, and a clock-extractor circuit 14 responsive to the voltage across the terminals of said resistor 13 and delivering an output signal which is applied to one of the electrodes of the laser 12.
Nevertheless, that device does not give complete satisfaction. In particular, it is limited in frequency and presents significant background noise.
It has therefore been abandoned in favor of dispositions of the type shown in FIG. 2 comprising a coupler 20 for extracting a portion of the input optical signal, a fast photodiode 22 for detecting said signal, and a clock extractor 24 for amplifying and processing the detected signal prior to applying it to a modulator 26 with a delay that is adjustable for synchronization.
The advantage of inline modulation has been demonstrated by Nakazawa et al. for soliton type transmission at 10 Gb/s over unlimited distance: 1! M. Nakazawa, E. Yamada, H. Kubota, K. Suzuki, "10 Gb/s soliton data transmission over one million kilometers", Electron. Lett., Vol. 27, p. 1270, 1991. Nakazawa's demonstration used a Mach-Zehnder interferometer type inline modulator on a substrate of LiNbO.sub.3.
It has been shown that with that type of configuration, the spacing between amplifiers can be increased by a factor of 2 (e.g. from 50 km to 100 km): 2! G. Aubin, T. Montalant, J. Moulu, B. Nortier, F. Pirio, J. B. Thomine, "Record amplifier span of 105 km in a soliton data demonstration experiment at 10 Gb/s over one million kilometers", Electron. Lett., Vol. 3, p. 217, 1995. In theory, that should enable a transoceanic system to be reduced substantially in cost. Never-the-less, inline use of that type of modulator is not very realistic since optical transmission through an LiNbO.sub.3 modulator depends strongly on the polarization state of the light.
Also, LiNbO.sub.3 modulators require a large control voltage and suffer from slow drift of the operating point (thereby degrading transmission).
In an attempt to remedy the problem of sensitivity to the polarization state of the light, proposals have been made to use two modulators on an LiNbO.sub.3 substrate in an orthogonal configuration: 3! M. Nakazawa, K. Suzuki, H. Kubota, E. Yamada, Y. Kimura, "20 Gb/s-3000 km straight line soliton transmission beyond Gordon-Hauss limit", Optoelectronics Conference (OEC'94), post-deadline proceedings, p. 6.
More recently, to eliminate the above-mentioned drawback of LiNbO.sub.3 modulators, proposals have been made to use an electroabsorbent modulator to provide inline signal modulation: 4! G. Aubin, E. Jeannery, T. Montalant, J. Moulu, F. Pirio, J. B. Thomine, F. Devaux, "20 Gb/s soliton transmission over transoceanic distance with a 105 km amplifier span", Electron. Lett., Vol. 31, No. 13, p. 1079, 1995.
Nevertheless, the above-mentioned devices do not give full satisfaction.
In particular, all of those known devices require an environment that is complex.